1. Field of the Invention
The present invention relates to a plasma etching method, and specifically to a plasma etching method including a step of etching a target object by use of plasma.
2. Description of the Related Art
In recent years, multi-layered structures are progressively developed for semiconductor devices, such as DRAMs (Dynamic Random Access Memory), to increase the degree of integration. In the process of fabricating a multi-layered structure, formation of contact plugs is repeatedly performed to connect layers on different levels. As a method for forming a contact plug, the following method is known. Specifically, an electrically conductive material, such as poly-crystalline silicon, is deposited on an inter-level insulating film to fill a contact hole by a CVD (Chemical Vapor Deposition) process. The conductive material is then subjected to an etching-back process to leave a part of the conductive material only in the contact hole.
In general, where poly-crystalline silicon, which is a mass of crystal grains, is etched, certain roughness appears on the etched surface. If such roughness is formed on a poly-crystalline silicon surface during an etching-back process, this roughness may be copied and left on an underlying film (for example, an inter-level insulating film) after the etching, and bring about defects in semiconductor devices.
As a technique for forming a smooth poly-crystalline silicon surface by etching-back, there has been proposed a plasma etching method (for example, Jpn. Pat. Appln. KOKAI Publication No. 9-232285) in which an etching gas containing a gas that can emit free sulfur in plasma is used to perform etching-back while forming a sulfur-containing material film on a poly-crystalline silicon surface. According to this proposed method, a gas that can emit free sulfur in plasma is a halogenated sulfur compound having a ratio of halogen atoms/sulfur atoms smaller than 6 (for example, S2F2, SF2, SF4, S2F10, S3Cl3, S2Cl2, or SCl2).
In the etching-back process described above, it is important not only to remove roughness of the poly-crystalline silicon surface, but also to attain a practically sufficient etching rate in light of an improvement in throughput. However, in the case of the method described in Jpn. Pat. Appln. KOKAI Publication No. 9-232285, it seems unlikely that the etching rate can be sufficiently high, because this method is a special method arranged to develop etching while forming a sulfur-containing material layer on a poly-crystalline silicon surface treated as an etching target. Further, the sulfur-containing material layer formed during etching needs to be removed by a heating process or ashing process at the end, thereby entailing an increase in the number of process steps.